1. Field of the Invention
This invention relates to a method and apparatus for maintaining multi-component sample gas constituents of a gaseous process stream in vapor phase so as to enable on-line characterization of these streams. More particularly, this invention relates to a method and apparatus for reducing the pressure and/or temperature of a stream extracted from a high pressure and/or high temperature industrial process so that condensable constituents in the unconditioned gas stream remain in the vapor phase and can be evaluated by standard process control instrumentation. This invention also relates to a specialized sample gas dilution apparatus that has been designed and modeled to provide uniform, precise levels of sample gas dilution and/or cooling. Finally, this invention relates to a method for predicting when a sample gas stream has been sufficiently cooled and/or diluted so that additional cooling or dilution is not required to avoid condensation when the conditioned sample is conveyed to an appropriate gas analyzer.
2. Description of Related Art
Gaseous process streams, such as those produced by synthesis gas production processes (e.g. coal, biomass and waste gasification), typically contain a significant proportion of vapor phase hydrocarbon species with dew points, at ambient pressures or above, that range from ambient temperature to process temperature. These gaseous process streams are often obtained at elevated conditions of pressure and/or temperature that exceed the operating limits of instruments available to perform on-line characterizations of these streams. In cases where on-line analysis is deemed necessary, representative sample streams must be extracted from the process and their pressure and/or temperature reduced to levels acceptable for these instruments. For some applications, difficulties and uncertainties in modifying the sample streams to conditions acceptable to on-line analyzers result in reliance on batch analyses performed on purposely condensed samples that are collected for remote laboratory analysis. Although batch sampling followed by remote analysis is often a simpler approach, results are delayed, and in cases where portions of the collected sample may be lost or chemically altered during collection, storage, and laboratory analysis procedures, this approach also may not be quantitatively or qualitatively accurate.
One conventional method for conditioning high temperature and/or high pressure gaseous process streams for analysis by analytical instrumentation is the use of syngas sampling trains in which depressurized syngas is passed through liquid impingers to trap and condense essentially all of the vapor-phase components in a suitable liquid carrier for subsequent analysis. When impingers are employed, the hot process gas may need to be cooled before it can be passed to the impinger train. Direct-contact heat exchangers are typically employed for this purpose and directly precede the impinger train. They are designed so that effective, intimate contact of the hot gas with cooled surfaces is maintained. The inherent weakness in this approach for syngas conditioning emerges when the gas is cooled to a temperature that is below the local dew point of one or more of its constituents (the dew points of water and hydrocarbon vapors are primarily determined by their local partial pressure within a heat exchanger). At the moisture and hydrocarbon species concentrations commonly found in gasifier process streams, transitions through these dew points are always encountered as the synthesis gas is cooled to ambient temperature. Thus, when using this conventional approach, some constituents of the process gas stream will always condense. When a dew point is reached, the water and/or at least a portion of the hydrocarbon species (tars, oils) condense and collect on cool surfaces. This condensation can degrade the efficiency of the heat exchanger, create cleanup, maintenance, and health issues, and provide the opportunity for free radicals in the condensed vapors to react and change in structure and concentration before analyses can be carried out. These tars and oils can also be challenging to remove from sample lines and traps.
For on-line analyses of gaseous process streams, a significant reduction of the temperature of an extracted gas sample stream is often required because the upper temperature limit of the on-line instrument(s) is often well below the lowest process temperature. Likewise, the pressure of the extracted sample stream must often be significantly reduced before it can be safely conveyed to the analyzer. Conventional pressure-reducing valves or orifices are commonly used to reduce gas pressures, and conventional contact heat exchangers are frequently used to reduce gas temperatures. However, as a gas sample cools, the potential for the condensation of vapor-phase components increases, particularly when interior portions of the heat exchanger are locally cooler than the condensation temperature for one or more of the constituents of the gas sample. As previously indicated, depending on the process and the analytical instruments used to characterize the process, the loss of vapor-phase constituents by condensation can result in plugged sample lines, delayed or inaccurate measurements, and failure of the gas analyzers. For this reason, syngas analyses have generally been limited to batch sample extraction methods that include built-in traps or reservoirs for collecting condensed hydrocarbons.
Accordingly, it is apparent that a better approach is needed to manage sample gas conditioning of gasification process streams to avoid condensation so that standard gas analyzers can be employed to quantify the various components of these gases.